1. Field of the Invention
The field of the present invention relates in general to semiconductor processing. More particularly, the field of the invention relates to a system and method for detecting and measuring semiconductor substrate properties, such as temperature, temperature uniformity, and emissivity, using optical pyrometry.
2. Background
In rapid thermal processing (RTP) of semiconductor device materials (such as a semiconductor wafer or other substrate), one of the critical process parameters is temperature. Repeatable, precise, and process-independent measurements of the wafer temperature are among the most important requirements of semiconductor processing equipment (such as RTP) in integrated circuit manufacturing.
Contact temperature sensors, such as thermocouples and thermistors, are commonly used to measure temperature. However, these sensors are not well-suited to many wafer processing environments. Such temperature sensors typically must be placed in contact with a wafer which may affect the uniformity of heating and expose the wafer to contaminants under certain conditions. In addition, it is difficult to achieve repeatable temperature measurement conditions due to inconsistent contact areas and other variations in heat transfer to the sensor.
As a result, noninvasive temperature measurement techniques, such as optical pyrometry, have been used in many RTP systems. Unlike temperature measurement using contact sensors, such as thermocouples and thermistors, temperature measurement using optical pyrometry does not require contact with the wafer and as a result does not expose the wafer to metallic contaminants during processing. Optical pyrometers may determine temperature based upon optical electromagnetic radiation (hereinafter "light") emitted from an object. Optical pyrometers typically use a high temperature optical fiber, light pipe, lens, or other light collecting device to transmit light to a light sensitive device that measures the flux density or intensity of the light emitted by the object. See, e.g., U.S. Pat. No. 4,859,079 to Wickerstein et al. and U.S. Pat. Nos. 4,750,139 and 4,845,647 to Dils et al., each of which is incorporated herein by reference. The temperature is then determined using Planck's equation which defines the relationship between the temperature of an object, the flux density of light being emitted from that object, and the light emitting characteristics of the object's surface (emissivity).
The advantages of optical pyrometry for RTP include its noninvasive nature and relatively fast measurement speed which is critical in controlling the rapid heating and cooling in RTP. However, accurate optical temperature measurement using pyrometry depends upon the accurate measurement of the flux density of light emitted from the wafer and upon the wafer's light emitting characteristics or emissivity. Emissivity is typically wafer dependent and depends on a range of parameters, including temperature, chamber reflectivity, the wafer material (including dopant concentration), surface roughness, and surface layers (including the type and thickness of sub-layers), and will change dynamically during processing as layers grow on the surface of the wafer.
Among other things, the emissivity of a semiconductor wafer depends upon the wavelength of light that is being measured. FIG. 1 is a graph of emissivity as a function of wavelength and temperature for a pure silicon wafer. As shown in FIG. 1, emissivity is temperature dependent and may vary greatly at wavelengths greater than the absorption band of silicon, which is slightly less than one and two tenths (1.2) micrometers. The effects of emissivity with respect to temperature on optical pyrometry for silicon can be minimized by using a sensor with maximum sensitivity within a range of about eight tenths (0.8) to about one and one tenth (1.1) micrometers wavelength, as indicated at 100 in FIG. 1. See also U.S. Pat. No. 5,166,080 to Schietinger et al. which is incorporated herein by reference. However, the wavelength/emissivity characteristic of a wafer will differ for doped silicon and other semiconductor materials such as gallium arsenide. In addition, the emissivity of a given wafer will typically change during processing as materials, such as silicon dioxide, are deposited on the wafer surface. Therefore, it is very difficult to control the effects of emissivity on temperature measurement based solely upon the wavelength of light that is used for pyrometry.
A short wavelength less than one (1) micrometer is often preferred for optical pyrometry since it provides certain benefits. For instance, a short wavelength improves the sensitivity of the temperature measurement which is based on Planck black-body emission. Sensitivity is defined as the fractional change in radiance per fractional change in temperature and from the equations for Planck black-body emission it can be shown that sensitivity is inversely proportional to wavelength. Therefore, shorter wavelengths are preferred for improved sensitivity in temperature measurement.
However, the ability to use a short wavelength of light in optical pyrometry is severely limited in many conventional rapid thermal processors due to interference from radiant energy heating sources. The heat needed for RTP is typically provided by a heating lamp module which consists of high intensity lamps (usually tungsten-halogen lamps or arc lamps). FIG. 2 illustrates a conventional RTP processing chamber, generally indicated at 200, using two banks of heating lamps 202 and 203 to heat a semiconductor wafer 204 through optical windows 206 and 207. An optical pyrometer 208 may be used to measure the wafer temperature by detecting the flux density of light within cone of vision 210. However, most radiant energy heating sources, including tungsten filament and arc lamp systems, provide their peak energy intensity at a wavelength of about one micrometer which interferes with optical pyrometer 208. Optical pyrometer 208 will detect light reflected off of wafer 204 from heating lamps 202 and 203 as well as light emitted from the wafer. This reflected lamp light erroneously augments the measured intensity of light emitted from the wafer surface and results in inaccurate temperature measurement.
Therefore, some conventional systems have used a longer wavelength of light to measure temperature so that the spectral distribution of the heating lamps has minimal overlap with the pyrometer's operating spectral band or wavelength. However, measuring the intensity of emitted light at a longer wavelength to reduce interference can lead to similar interference problems from light emitted by hot chamber surfaces which, at longer wavelengths, becomes significant enough to cause errors in the measured light intensity. For instance, quartz, which is typically used in RTP processing chambers, re-emits light predominantly at longer wavelengths (typically greater than 3.5 micrometers). An alternative approach is to measure and compensate for reflected light. Two optical pyrometers may be used--one for measuring the light from the lamps and one for measuring the light from the wafer. The strength of the characteristic AC ripple in light emanated from the lamp can be compared to the strength of the AC ripple reflected from the wafer to determine the wafer's reflectivity. This, in turn, can be used to essentially subtract out reflected light in order to isolate the emitted light from the wafer for determining temperature using Planck's equation. See, e.g., U.S. Pat. No. 5,166,080 to Schietinger et al. However, such systems may require complex circuitry to isolate the AC ripple and perform the calculations that effectively eliminate reflected light. Such systems also require an additional light pipe and other components.
FIG. 3A illustrates an RTP processing chamber, generally indicated at 300, with a single set of heating lamps 302 for heating wafer 304. Typically, wafer 304 is supported by low thermal mass pins (not shown). An optical pyrometer 308 may be placed behind the wafer opposite the heating lamps 302. This reduces, but does not eliminate, the interference from lamps 302. In addition, wafer 304 may remain at least partially transparent to lamp radiation in the infrared region (beyond 1.5 micrometers) at lower wafer temperatures (below 600.degree. C.) so pyrometer 308 may still be affected by lamp radiation or radiation re-emitted from quartz that is transmitted through wafer 304. FIG. 4 is a graph of silicon wafer transmission as a function of wavelength at a temperature of twenty five degrees Celsius (25.degree. C.). As can be seen, silicon transmission is greatly reduced for wavelengths less than about one and two hundredths (1.02) micrometers. Therefore, transmission problems can be substantially eliminated by using short wavelengths (which also provide advantages for emissivity and sensitivity as described above), but interference from heating lamps 302 is also greatest at such wavelengths.
FIG. 3B illustrates an alternative single-sided lamp RTP processing chamber, generally indicated at 350. This processing chamber 350 includes a backside shield 352 for preventing deposition on the backside of wafer 354. This is desirable in many processing applications including chemical vapor deposition (CVD). In contrast, systems that use only low thermal mass pins to support the wafer allow material to be deposited everywhere on the wafer, including the backside which is exposed. Preventing such backside deposition usually simplifies the overall semiconductor device fabrication process. As shown in FIG. 3B, such systems may use an optical pyrometer 356 aimed at the backside shield to measure the approximate wafer temperature. While the shield eliminates wafer transmission problems, it also reduces the accuracy of temperature measurement since it is the temperature of the shield, and not the wafer, that is actually being measured.
For both double-sided and single-sided lamp RTP systems, additional problems may be introduced if the optical pyrometer samples emitted light through a window or lens that is exposed to the processing chamber. During certain processes, deposits may form on the window or lens which inhibit the transmission of light to the optical pyrometer. The optical pyrometer will sense less light due to the deposits and produce a temperature measurement that is lower than the actual temperature of the wafer. This may cause the lamp control system to erroneously increase the heat provided to the chamber to compensate for the falsely detected low temperature measurement. As the heat is increased, deposits may accelerate leading to thermal runaway conditions.
Another approach to compensating for the effects of emissivity on temperature measurement is to use a reflective cavity. A processing chamber may be designed to reflect emitted light from remote, highly reflective walls. The emitted light is radiated diffusely and reflected over the entire wafer numerous times. The effective emissivity of the wafer in such a system is determined by integrating the reflected light intensity over the wafer surface. By using a high aspect ratio for the reflective cavity, the effective emissivity of a wafer is increased toward unity which helps eliminate the effects of edges, device patterns on the wafer, and backside roughness. However, such an approach restricts chamber geometry and may not be practical in cold wall RTP chambers. The reflective cavity approach typically treats the wafer as an extension of the chamber wall. However, a wafer should not make contact with, or be too closely spaced to, a cold wall in RTP systems to avoid uneven cooling at the wafer edges. Further, reflective walls may interfere with rapid cooling in RTP systems which may require walls that rapidly absorb energy rather than reflect it. In addition, deposits may form on reflective walls during some processes which may diminish reflectivity and thereby introduce error into the temperature measurement.
Another approach is to place a small black-body enclosure around the tip of an optical sensor. The black-body enclosure heats up to the approximate temperature of the surrounding environment and emits light into the optical sensor that is proportional to the temperature of the black-body enclosure. However, such an approach measures the temperature of the black-body emitter and not the wafer. As with thermocouples, such an approach typically requires that the temperature sensor be placed in contact with the wafer, and repeatable accurate temperature measurements are difficult to achieve under actual processing conditions.
Yet another approach for measuring wafer temperature and compensating for emissivity uses an infrared laser source that directs light into a beam splitter. From the beam splitter, the coherent light beam is split into numerous incident beams which travel to the wafer surface via optical fiber bundles. The optical fiber bundles also collect the reflected coherent light beams as well as radiated energy from the wafer. In low temperature applications, transmitted energy may be collected and measured as well. The collected light is then divided into separate components from which radiance, emissivity, and temperature may be calculated. See, e.g., U.S. Pat. No. 5,156,461 to Moslehi et al. It is a disadvantage of such systems that a laser and other complex components are required. Such systems, however, are advantageous because they may provide measurements of wafer temperature at multiple points along the wafer surface which may be useful for detecting and compensating for temperature nonuniformities.
Advanced fabrication processes demand uniform temperature across the wafer with gradients preferably less than plus or minus two degrees Celsius (.+-.2.degree. C.) to provide for uniform processing and to avoid thermal induced stress which may cause crystal slip in the wafer. However, RTP typically requires low thermal mass to allow for rapid heating and cooling. Such systems use a cold-wall furnace with radiant energy sources to selectively heat the wafer. While this allows rapid heating and cooling, the temperature uniformity becomes sensitive to the uniformity of the optical energy absorption as well as the radiative and convective heat losses of the wafer. Wafer temperature non-uniformities usually appear near wafer edges because radiative heat losses are greatest at the edges. During RTP the wafer edges may, at times, be several degrees (or even tens of degrees) cooler than the center of the wafer. The temperature nonuniformity may produce crystal slip lines on the wafer (particularly near the edge). Slip lines are collections of dislocations in the crystal lattice structure of the silicon caused by unequal movement of atomic planes due to thermally induced stress. This may result in the formation of electrically active defects which degrade the circuitry and decrease yield. Therefore, a system for detecting and correcting temperature differences across the wafer is often required. In particular, accurate, multi-point optical pyrometry is desirable for detecting temperature differences which can then be corrected.
Correction or compensation for temperature nonuniformities has been provided using several techniques. One approach is to use a multi-zone lamp system arranged in a plurality of concentric circles. The lamp energy may be adjusted to compensate for temperature differences detected using multi-point optical pyrometry. However, such systems often require complex and expensive lamp systems with separate temperature controls for each zone of lamps. Another approach has been to place a ring of material (such as silicon or the like) around, and in contact with, the periphery of the wafer. The ring provides extra thermal insulation to retain heat at the periphery of the wafer, but typically does not offer sufficient flexibility over a wide range of temperatures.
What is needed is an RTP processing chamber with an accurate optical pyrometry system for measuring semiconductor substrate temperature. Preferably such a system would provide optical pyrometry at short wavelengths without interference from direct lamp light or reflected or re-emitted light from chamber surfaces. Further, such a system would preferably be emissivity insensitive to allow accurate temperature measurement in a variety of processes using different semiconductor materials, dopants, and layers and would not require complex emissivity and reflectivity measurement and compensation systems as do many conventional approaches. In addition, such a system would preferably allow an optical pyrometer to measure the intensity of emitted light without interference from coatings or other obstructions on windows which may lead to thermal runaway conditions.
What is also needed is an RTP processing system and method for preventing undesired backside deposition. Preferably, such a system would prevent backside deposition on a semiconductor substrate while still allowing direct multi-point temperature measurement. What is also needed is a system and method for detecting and correcting temperature differences across a semiconductor substrate due to edge losses without requiring complex multi-zone lamp systems. Preferably, each of these features would be combined in a single, cost-effective RTP processing system and method.